The present invention relates to a variable-capacity control apparatus to be employed in conjunction with a refrigerating cycle which uses carbon dioxide as the coolant and includes a variable-capacity compressor provided with a swash plate tiltably secured to a drive shaft and a piston caused to move reciprocally inside a compression space as the drive swash plate rotates to vary the capacity for the coolant flowing through the refrigerating cycle by varying the piston stroke in correspondence to the tilt angle of the drive swash plate based upon the difference between the pressure in the compression space and the piston back pressure.
The pressure control valve used in the variable-capacity swash plate compressor disclosed in Japanese Unexamined Patent Publication No. H 5-99136 includes a first control valve that implements open/close control on the communication between an outlet chamber and a crank case, a second control valve that implements open/close control on the communication between the crank case and an intake chamber, a transmission rod that engages the first and second control valve in operation, an electromagnetic actuator that moves the transmission rod and the pressure-sensitive member (such as a diaphragm or a bellows) that engages the second control valve in operation by sensing the pressure within the intake chamber.
The control valve for a variable-capacity compressor disclosed in Japanese Unexamined Patent Publication No. H 9-268974 comprises a valve element that opens/closes an air supply passage communicating between an outlet pressure area and a crank case, a pressure-sensitive unit that is linked to one side of the valve element via a pressure-sensitive rod to achieve interlocked operation and is housed within a pressure-sensitive chamber communicating with an intake pressure area to apply a force to the valve element along the direction in which the degree of openness of the air supply passage is reduced as the pressure in the intake pressure area rises, a solenoid unit that is linked to the other side of the valve element via a solenoid rod to achieve interlocked operation and applies a load to valve element along the direction in which the degree of openness of the air supply passage is reduced as the solenoid becomes excited and a means for forced opening that applies a force to the valve element along the direction in which the air supply passage is forcibly opened as the solenoid becomes demagnetized, with the valve element and the pressure-sensitive unit linked with each other in such a manner that the contact between the valve element and the pressure-sensitive unit can be established/cut off freely.
When the pressure within the pressure-sensitive chamber enters a high intake pressure condition while the solenoid at the solenoid unit remains demagnetized, the pressure-sensitive unit becomes displaced along the direction in which the degree of openness of the air supply passage is reduced. At this time, the force applied by the means for forced opening to the valve element works in the opposite direction from the direction of the displacement of the pressure-sensitive unit, thereby causing the pressure-sensitive unit and the valve element to separate from each other and sustaining the valve element at its maximum opening position. It is to be noted that the publication above discloses that the pressure-sensitive unit is constituted of a bellows and also discloses that it may alternatively be constituted of a diaphragm.
However, when utilizing a control valve having a diaphragm or a bellows to constitute the pressure-sensitive element at the pressure-sensitive unit as in the examples referred to above in conjunction with a refrigerating cycle that uses carbon dioxide as the coolant with the pressure inside the refrigerating cycle reaching a level as high as approximately 10 times that in a refrigerating cycle using freon as the coolant as in the prior art, a problem arises in that it is difficult to achieve a satisfactory degree of pressure withstanding performance at the pressure-sensitive element. There is another problem in that since it is necessary to apply the electromagnetic force of the electromagnetic actuator provided to drive the valve against a high pressure, the size of the electromagnetic actuator itself is bound to be large.
Accordingly, an object of the present invention is to provide a variable capacity control apparatus for a refrigerating cycle that implements reliable variable-capacity control while achieving a satisfactory level of coolant pressure withstanding performance against the pressure in the refrigerating cycle using carbon dioxide as the coolant without having to increase the size of the pressure control valve.
In order to achieve the object described above, a refrigerating cycle that uses carbon dioxide as a coolant, comprising at least a variable-capacity compressor having at least a cylinder block, a drive shaft provided inside the cylinder block, a drive swash plate that rotates together with the drive shaft and whose angle of inclination relative to the drive shaft can be varied freely, a plurality of cylinders provided within the cylinder block, each having an axis parallel to the drive shaft, a plurality of pistons slidably provided at the cylinders and caused to make reciprocal movement within the cylinders as the drive swash plate rotates, compression spaces defined by the cylinders and the pistons, a crank case formed on a non-compression side of the pistons, an intake chamber that communicates with the compression spaces during the intake phase of the pistons and an outlet chamber that communicates with the compression spaces during the compression phase of the pistons, a radiator that cools the coolant having been compressed at the variable-capacity compressor, a means for expansion that expands the coolant having been cooled by the radiator and an evaporator that evaporates the coolant having been expanded by the means for expansion, is further provided with a variable-capacity mechanism that includes at least a low pressure chamber that communicates with the intake chamber, a high pressure chamber that communicates with the outlet chamber, a pressure adjustment chamber that communicates with the crank case, a low pressure side communicating port provided between the pressure adjustment chamber and the low pressure chamber, a high pressure side communicating port provided between the pressure adjustment chamber and the high pressure chamber, a valve element that opens/closes the low pressure side communicating port and, at the same time, opens/closes the high pressure side communicating port, an electromagnetic coil that generates an electromagnetic force, a plunger that is slidably inserted at the electromagnetic coil and is moved by the electromagnetic force imparted by the electromagnetic coil to cause the valve element to move and a spring that applies a force to the valve element along the direction opposite from the direction in which the valve element is caused to move by the plunger, a pressure sensor that detects the pressure on a low pressure line extending from the outlet side of the means for expansion to the intake side of the variable-capacity compressor in the refrigerating cycle and a means for control that controls the electromagnetic coil to move the valve element along the direction in which the pressure adjustment chamber and the low pressure chamber come into communication with each other and the pressure adjustment chamber becomes cut off from the high pressure chamber if the low level pressure detected by the pressure sensor is higher than a target pressure and to move the valve element along the direction in which the pressure adjustment chamber becomes cut off from the low pressure chamber and the pressure adjustment chamber and the high pressure chamber come into communication with each other if the low level pressure is lower than the target pressure are provided.
According to the present invention provided with the pressure sensor that detects the pressure on the low pressure side of the refrigerating cycle using carbon dioxide as the coolant, the valve element is caused to move along the direction in which the value of the pressure detected by the pressure sensor is made to match the target pressure, e.g., along the direction in which the low level pressure is lowered if the detected value is higher than the target pressure and along the direction in which the low level pressure is raised if the detected value is lower than the target pressure, by controlling the electromagnetic coil. Thus, it is not necessary to include any portion with a low pressure withstanding capability such as the low level pressure detection unit in the prior art, thereby achieving higher pressure withstanding performance against the pressure in the refrigerating cycle.
In addition, according to the present invention, it is desirable to ensure that the valve element is set at a position at which it cuts off communication between the low pressure chamber and the crank case and allows the high pressure chamber and the pressure adjustment chamber to communicate with each other when no power is supplied to the electromagnetic coil and that the valve element is caused to move along the direction in which the low pressure chamber and the pressure adjustment chamber come into communication with each other and the high pressure chamber becomes cut off from the pressure adjustment chamber by the electromagnetic force imparted by the electromagnetic coil. By minimizing the compressor outlet capacity when no power is supplied to the electromagnetic coil, smoother operation is achieved during the initial stage of compressor startup.
It is also desirable to form a small hole at the valve element, through which the pressure adjustment chamber and the low pressure chamber are allowed to communicate with each other when the valve element has cut off the pressure adjustment chamber from the low pressure chamber. Since this allows a small quantity of coolant to flow toward the low pressure side from the crank case, an increase in the temperature inside the crank case is prevented.
Furthermore, the valve element includes a valve element main body provided within the pressure adjustment chamber and a guide unit extending from the high pressure side communicating port and passing through the high pressure chamber, which communicates the force imparted by the spring to the valve element main body, with a pressure, the level of which is equal to the pressure level in the low pressure chamber, supplied into a spring housing chamber housing the spring and the guide unit, pneumatically cutting off the spring housing chamber from the high pressure chamber. As a result, the low level pressure is applied to the two sides of the valve element, allowing the valve element to engage in even smoother operation compared to a valve element having different pressures applied to the two sides thereof. Thus, the operating load on the valve element is reduced and, ultimately, the electromagnetic coil can be miniaturized.
It is desirable that the control signal provided to the electromagnetic coil be a duty ratio signal with its maximum voltage restricted to a predetermined voltage level by a constant voltage circuit. By sustaining a constant voltage with the constant voltage circuit and adjusting the power level in conformance to the duty ratio when the refrigerating cycle is installed in a vehicle in which the voltage at the battery power source constituting the source fluctuates greatly, the extent to which such voltage fluctuations affect the operation of the refrigerating cycle can be minimized so that the movement of the valve element is controlled in a stable manner. Moreover, impact noise which will occur as the valve element comes in contact with the valve seat due to excessive electromagnetic force can be suppressed.
The valve stroke quantity representing the distance between the position at which the valve element blocks the low pressure side communicating port and the position at which the valve element blocks the high pressure side communicating port should be preferably set to 1 mm or smaller. By reducing the distance over which the valve element travels, the high pressure chamber and the low pressure chamber are both allowed to establish/cease communication with the pressure adjustment chamber quickly.
Moreover, it is desirable that the target pressure be calculated in conformance to the heat load environment of the refrigerating cycle to ensure that the optimal target pressure corresponding to specific air-conditioning conditions is set.